1. Field of the Invention
This invention relates to a process, apparatus and materials for use in the detection of surface defects in manufactured parts. More particularly, the invention relates to defect detection methods which employ fluorescent dye penetrants to indicate the presence of defects in a part being tested.
2. Description of Background Art
The use of dye penetrants to detect small flaws in critical manufactured components such as jet engine turbine blades is well known. Typically, flaw detection by the dye penetrant method requires the following sequence of steps.
First, the object to be tested is cleaned thoroughly, using a vapor degreaser, for example. Next the object is immersed in a dye penetrant solution which contains a solvent and a dye, typically one which fluoresces brightly when irradiated by long-wave ultraviolet radiation, in the approximate range of 3000 .ANG. (300 nm) to 4000 .ANG. (400 nm). Alternatively, dye penetrant may be sprayed upon the surface of the object to be tested. Third, excess dye penetrant solution is removed from the surface of the part with the aid of another solution, an emulsifier. The part is then washed and dried. Finally, the part under test is illuminated with a source of long-wave ultraviolet radiation. Dye penetrant which has been entrapped in samll voids such as cracks, seams or porous areas fluoresces brightly when illuminated by the ultraviolet radiation source, providing a visual indication of small defects which would otherwise escape visual detection. Sometimes a developer solution is applied to the surface of the part after that surface has been cleansed of excess dye penetrant solution. The function of the developer solution is to draw up to the surface of the part, by capillary action, dye which has been entrapped in voids some distance below the surface. This makes the dye and therefore the void more readily visible.
Fluorescent dye penetrant solutions commonly used are either water soluble, or oily, non-water soluble. The latter type penetrant solution requires a hydrocarbon solvent such as mineral spirits to remove excess penetrant solution from the surface of the object to be tested. Alternatively, an emulsifier may be applied to the part being tested, rendering the excess oily penetrant solution soluble in water, which may then be used to cleanse the surface of the part.
The dye penetrant inspection processes just described are generally effective in performing their intended function of detecting small surface flaws in manufactured parts. However, existing dye penetrant testing processes all have certain drawbacks which limit their usefulness for "in-process" inspection applications. For example, the immersion method of applying dye penetrant to a part to be tested often requires that the part be pre-heated, and that the part remain immersed for a substantial period. Thus the immersion method of penetrant testing can be time consuming. Also, large and cumbersome parts require large immersion tanks and large volumes of dye penetrant solution.
Existing oily penetrants have a characteristic high mobility which causes ultra-small penetrant entrapments to migrate and thin out to the point where their visibility is greatly reduced. Furthermore, oily penetrants are incompatible with soldering fluxes and therefore cannot be conveniently used for the inspection of defective solder joints. On the other hand, various delicate electronic components cannot tolerate water contact. Therefore, circuit boards containing these components cannot be tested using water-soluble penetrant solutions.
The spray method of applying dye penetrant also has limitations The volatile components used in the spray often present fire and health hazards. Also, uncontrollable overspray can present a problem. The present invention overcomes some of the problems inherent in the existing methods of dye penetrant flaw detection.